1. Technical Field
The invention relates to a color signal processing apparatus, a color conversion apparatus, a color signal processing method, a color conversion method, a computer-readable medium and a computer data signal embedded with a program.
2. Related Art
There is a case that, in an output device for outputting a color image or the like using plural coloring materials, a total amount of the coloring materials used may be limited in order to, for example, maintain a necessary image quality or reduce the load on the output device. For example, where coloring materials of C (cyan), M (magenta), Y (yellow), and K (black) are used, each coloring material amount takes a value of 0% to 100%, and the total coloring material amount takes a value of 0% to 400%. For example, where the total coloring material amount of the output device is limited to 300% (total amount limit value), it is necessary to convert a color having a total coloring material amount larger than 300%, into another color having a total coloring material amount equal to or less than the total amount limit value.
For example, JP Hei.9-247471 A describes that, if a table value of a color conversion table does not satisfy a condition that the total amount is equal to or less than 250%, the table value is changed to a value that satisfies this condition so that a total amount obtained as a result of a color conversion does not exceed the total amount limit value. Also, JP Hei.9-261492 A describes that, if the total amount of inks used exceeds a total amount limit value, C, M, and Y values are decreased while a K value is fixed so as to enable output within the total amount limit value. JP 2000-25274 A describes allowing use of only colors which satisfy a total amount limit value from the beginning. A coloring material amount limiting function is determined by obtaining calorimetric values of the colors satisfying the total amount limit value, and outputs within the total amount limit value are made using this coloring material amount limiting function. JP 2003-125225 A describes limiting a total coloring material amount to be equal to or less than a total amount limit value by increasing or decreasing respective coloring material amounts uniformly before a tone correction so that density values after the one correction satisfy the total amount limit value. Furthermore, JP 2004-147257 A describes generating a color prediction model that satisfies a total amount limit value by changing output values of color patches, which are used in generating the color prediction model, to values of colors that satisfy the total amount limit value.
FIG. 14 is a schematic chart illustrating an example of a color gamut of an output device to which total amount limitation is imposed. Each of JP Hei.9-247471 A, JP Hei.9-261492 A, JP 2000-25274 A, JP 2003-125225 A and JP 2004-147257 A describe such techniques that if a total amount limit value is exceeded, output is made by changing a color so as to satisfy the total amount limit value or using a color gamut that satisfies the total amount limit value. An example of the color gamut in those cases is shown in FIG. 14, which is a section (L-C plane) of the color gamut take along a certain hue. Methods for determining a contour of a color gamut that satisfies the total amount limit value are described in JP 2005-63093 A (corresponding to US 2005/0062992 A) and JP 2006-352475 A, for example.
A point P in FIG. 14 represents a color on a boundary of limitation based on a total amount limit value. The contour of the color gamut is bent at the point P. For example, when a color is varied from a maximum chroma color T toward the low-lightness side along the contour of the color gamut, the color would pass through points indicated by black circles if there were no total amount limitation. However, because of the total amount limitation, colors on the contour of the color gamut which are indicated by the broke line on the low-lightness side of point P are not reproduced, and the color is varied so as to pass white circles which are located on a contour of the color gamut represented by the solid line. As a result, a step in color occurs at the point P.
FIG. 15 is a detailed chart illustrating an example of a color conversion into the color gamut of the output device to which the total amount limitation is imposed. As described above with reference to FIG. 14, the bending point P exists in the color gamut of the output device to which the total amount limitation is imposed. For example, an influence of this phenomenon appears in a process of converting a color outside the color gamut of the output device into a color in the color gamut. FIG. 15 shows an example in which colors indicated by black circles, which are located outside the color gamut and are spaced at regular intervals, are converted into colors on the color contour of the color gamut, which have minimum color differences. The resulting colors produced from the original colors, which are originally spaced at regular intervals, are dense in tone around the point P and, therefore, are sparse in tone on the low-lightness/low-chroma side. Furthermore, the tendency of color variation is changed at the point P.
FIG. 16 is a chart illustrating another example of a color conversion into the color gamut of the output device to which the total amount limitation is imposed. FIG. 16 shows an example in which a certain input color gamut is changed, in its entirety, to the color gamut (output color gamut) of the output device. In this color conversion, colors on the contour of the input color gamut are converted into colors on the contour of the output color gamut, and colors which are located on each conversion direction are converted at the same conversion ratio. As a result, colors (black circles) on the contour of the input color rage are converted into colors (white circles) on the contour of the output color rage, and the tendency of color variation is changed at the point P. At the same time, colors (black triangles) inside the contour of the input color gamut that are arranged at regular intervals are converted into colors (white triangles) that are arranged so as to conform to the contour shape of the output color gamut. Therefore, in this case, the color conversion changes a tendency of color variation of even colors inside the contour of the color gamut.
FIG. 17 is a graph showing an example of a variation of the total amount caused by the total amount limitation. As described above, the bending point P appears because the total amount limitation is started at the point P. For example, when a color is varied from the maximum lightness point in FIG. 14 along the contour of the color gamut, the total amount varies as shown in FIG. 17. The coloring material total amount increases as the color varies up to the point P through the point T. The coloring material total amount reaches the total amount limit value at the point P and cannot increase any more. Therefore, the coloring material amount increasing/decreasing method needs to be changed at the point P, which results in that the tendency of color variation is changed at the point P.
For example, JP 2005-236802 A describes the following techniques. In a color conversion table for converting an input color in an input color space into an output color in an output color space, an output color is subjected to a smoothing process or a K (black) component, for example, of output color components is subjected to the smoothing process, and the other color components are predicted based on the resulting K component and the input color. In generating the color conversion table, a total amount limit value is subjected to the smoothing process or a K (black) component of the output color components is further subjected to the smoothing process, and an output color is re-calculated based on the input color. These techniques smooth out a bending point such as the point P and solves such problems as a sharp variation in tone and a drastic variation in the tendency of color variation. In JP 2005-236802 A, although the input color is once converted into the output color, if the total limit value is exceeded, conversion from the input color into another output color is performed again.